Metallized opaque film with barrier properties

ABSTRACT

The invention relates to a metallised, coextruded, multi-layered, biaxially orientated polypropylene multi-layer film comprising a base layer containing vacuoles. Said base layer containing vacuoles is covered by one or several layers and the thickness of said covering layer or layers is, in total, at least 3 μm and the film on the outer surface of said covered layer or layers are metallised and the metallised film has a water vapour permeability of =0.5 g/m 2 *day at 38° C and 90 % relative air humidity and oxygen permeability of =50 cm 3 /m 2 *day*bar at 23° C. and 50% relative air humidity.

The present invention relates to a metallized opaque polypropylene filmand a method for its manufacture.

Biaxially oriented polypropylene films (boPP) are currently used aspackaging films in greatly varying applications. Polypropylene films aredistinguished by many advantageous usage properties such as hightransparency, gloss, barrier to water vapor, good printability,rigidity, piercing resistance, etc. In addition to the transparentfilms, opaque polypropylene films have been developed very successfullyin past years. The special appearance (opacity and degree of whiteness)of these films is especially desirable for certain applications. Inaddition, opaque films offer a higher yield to the user because of thereduced density of these films.

In spite of these manifold favorable properties, there are still areasin which the polypropylene film must be combined with other materials inorder to compensate for specific deficits. In particular for bulkproducts which are sensitive to moisture and oxygen, polypropylene filmshave not been successful until now as the sole packaging material. Forexample, in the field of snack packaging, both the water vapor barrierand also the oxygen barrier play a decisive role. With water absorptionof only 3%, potato chips and other snack items become so sticky that theconsumer finds them inedible. In addition, the oxygen barrier mustensure that the fats contained in the snack items do not develop arancid taste through photooxidation. These requirements are notfulfilled by polypropylene film alone as the packaging material.

The barrier properties of polypropylene films having avacuole-containing base layer are even more problematic, since in thesetypes of films the vacuoles in the base layer additionally impair thewater vapor barrier. For example, the water vapor barrier of atransparent biaxially oriented polypropylene film of 25 μm isapproximately 4.4 g/m²*day at 38° C. A comparable barrier value isreached with an opaque film having a vacuole-containing base layer onlyfrom a thickness of 35 μm. The oxygen barrier is completely insufficientfor many applications both in transparent and in opaque polypropylenefilms (>2000 cm³/m²*day*bar).

Improving the barrier properties of boPP by metallization, by which boththe water vapor permeability and also the oxygen permeability aresignificantly reduced, is known. Opaque films are typically not used inmetallization, since their barrier is significantly worse withoutmetallization than that of a transparent film. The barrier of themetallized films is better the better the barrier of the base filmbefore the metallization is. For example, the oxygen permeability of atransparent 20 μm boPP film may be reduced through metallization andlamination with a further 20 μm transparent film to approximately 40cm³/m²*day*bar (see VR Interpack 99 Special D28 “Der gewisse Knack [thespecial snap]”).

In some applications, the good barrier, as is known from transparentmetallized films, is to be combined with the special opaque appearanceof the vacuole-containing films, i.e., a metallized opaque barrier filmis to be provided. In order to compensate for the known poor barrierstarting values of opaque films, barrier coatings, made of PVOH, PVDC,or EVOH, for example, are applied before the metallization, in order toreduce the permeability of the substrate to be metallized. Aftermetallization on the coating, outstanding barrier values may be achievedeven in opaque films. However, these achievements of the object are verycostly, since two costly finishing steps are necessary.

In some applications, boPP films are also metallized only inconsideration of the visual impression. In this case, the impression ofa high quality package is to be given to the consumer, without a betterbarrier actually existing. In these cases, the requirements for themetallized film are comparatively non-critical. The metallized film mustonly have a uniform appearance and adequate metal adhesion. The barrierachieved plays no role and is only insignificantly improved by themetallization.

DE 39 33 695 describes a non-sealable film made of a base layer made ofpolypropylene and at least one covering layer, which is synthesized froma special ethylene-propylene copolymer. This copolymer is distinguishedby an ethylene content of 1.2 to 2.8 weight-percent and a distributionfactor of >10 and a melting enthalpy of >80 J/g and a melt flow index of3 to 12 g/10 minutes (21.6 N and 230° C.). According to the description,the properties of the copolymer must be kept within these narrow limitsto improve the printability and the visual properties. This publicationrelates overall to transparent films.

The present invention is based on the object of providing an opaque filmhaving good barriers to oxygen and water vapor. Of course, the typicalusage properties of the film in regard to its employment must also bemaintained.

The object on which the present invention is based is achieved by acoextruded multilayered, biaxially oriented polypropylene multilayeredfilm, which is metallized on at least one outer surface of thecoextruded film and has a vacuole-containing base layer, thisvacuole-containing base layer being covered by one more layers and thethickness of this cover layer or layers being a total of at least 3 μmand the metallized film having a water vapor permeability <0.5 g/m²*dayat 38° C. and 90% relative ambient humidity and an oxygen permeabilityof ≦50 cm³/m²*day*bar.

As defined in the present invention, the base layer is the layer of thefilm which makes up more than 40%, preferably more than 50% of the totalthickness of the film. In a possible embodiment, a cover layer may beapplied directly to the base layer, which then forms the first coveringlayer of the film. In this embodiment, the thickness of the firstcovering layer is at least 3 μm, preferably 4 to 8 μm. In a furtherembodiment, further layers may additionally be attached between thisfirst covering layer and the vacuole-containing base layer, which thenform one or more first intermediate layers. Covering layers form theexternal layers of the non-metallized coextruded film. In thisembodiment, the total thickness of covering layer and intermediate layeris at least 3 μm, preferably 4-8 μm, the minimum thickness of thecovering layer generally being 0.5 μm and therefore the correspondingminimum thickness of the first intermediate layer being 2.5 μm. A secondoptional covering layer on the diametrically opposite side of the baselayer may be applied directly to the base layer. Furthermore, there areembodiments in which both covering layers are applied to theintermediate layers of the film.

It was found that the film surprisingly has an outstanding barrier afterthe metallization if the base layer of the film contains vacuoles and iscovered by one or more additional layers and this layer or layers has atotal thickness of at least 3 μm. The metallization is located on theouter surface of the first covering layer.

Surprisingly, this measure improves the barrier of the filmsignificantly after metallization, although no special barrierproperties could be detected at the non-metallized opaque film and noother special measures, such as coatings, were used to improve thenon-metallized substrate.

The opaque film according to the present invention is distinguished byoutstanding barrier values, which have not been implemented previouslyfor opaque films. The water vapor permeability of the opaque metallizedfilm according to the present invention is generally ≦0.5 g/m²*day at38° C. and 90% relative ambient humidity, preferably in a range from0.05 to 0.3 g/m²*day. The oxygen permeability is preferably ≦50cm³/m²*day*bar, preferably 5 to 30 cm³/m²*day*bar, particularly 5 to 25cm³/m²*day*bar.

The outer coextruded covering layer of the opaque film to be metallizedmay be synthesized from isotactic propylene homopolymers made ofethylene homopolymers or from mixed polymers of propylene or ethylene,which have a low comonomer content. In general, the first covering layerto be metallized contains at least 80 weight-percent, preferably 85 to<100 weight-percent, particularly 95 to 99 weight-percent of theabove-mentioned polymers or mixtures thereof.

Suitable propylene homopolymers are isotactic propylene homopolymerswhich are synthesized 100 weight-percent from propylene units and have amelting point of 160° C. or higher, preferably 162° C. In general, thesepolypropylene homopolymers have a melt flow index of 1 to 10 g/10minutes, preferably 2 to 8 g/10 minutes, at 230° C. and a force of 21.6N (DIN 53735). Isotactic propylene homopolymers having an atacticproportion of 10 weight-percent, preferably <5 weight-percent, representpreferred propylene polymers for the first covering layer. The specifiedweight percents relate to the particular polymer. In a furtherembodiment, the isotactic polypropylene may be a highly isotacticpolypropylene having an isotacticity of over 95%. Materials of this typeare known per se in the related art and are also referred to as HCPP(high crystallinity polypropylene). If necessary, an isotacticpolypropylene which is manufactured using a metallocene catalyst may beselected. These metallocene polypropylenes are preferably distinguishedby a narrow molecular weight distribution (Mw/Mn<2).

Suitable polyethylenes are, for example, HDPE or LDPE polymers which areused in a way known per se as layers to be metallized in boPP films.

Mixed polymers having a low comonomer content generally contain thecomonomer(s) in a quantity of <3 weight-percent, preferably 0.1 to 2.5weight-percent. This comonomer component is to be incorporateddistributed as well as possible in the chains of the basic polymer,because of which these mixed polymers are also referred to as randomcopolymers or random terpolymers. In particular, propylene copolymershaving a low ethylene content of <2.5 weight-percent and a melting pointof 150 to 165° C. are preferred. These materials are known per se andare also described as “minicopo” because of their relatively lowethylene content, for example, in EP 0 361 280 or DE 39 33 695. Forexample, these propylene-ethylene copolymers have an ethylene content of1.2 to 2.8 weight-percent, particularly 1.2 to 2.3 weight-percent,preferably 1.5 to <2 weight-percent, and a melting point of 150 to 155°C. and a melting enthalpy of 90 to 100 J/g and a melt flow index of 3 to15 g/10 minutes, preferably 3 to 9 g/10 minutes (230° C., 21.6 N DIN 53735). Furthermore, propylene-ethylene copolymers may be used which havean ethylene content of <1 weight-percent, preferably 0.05 to 0.7weight-percent, which are described, for example, in U.S. Pat. No.5,958,566. In principle, propylene copolymers having a low butylenecontent of less than 2.5 weight-percent are also usable. These polymersare also known per se, have already been described in the literature,and are commercially available. Reference is hereby expressly made tothe cited publications and the description of these polymers in thesepublications.

In addition to this main component, the first covering layer may containtypical additives such as antiblocking agents, stabilizers, and/orneutralization agents in the particular effective quantities. In regardto the metallization, additives which impair the ability to bemetallized should not be contained in the covering layer or should onlybe contained in the smallest quantities. This applies to migratinglubricants or antistatic agents, for example.

To improve the metal adhesion, the surface of the first covering layeris generally subjected in a way known per se to a method for elevatingthe surface tension using corona, flame, or plasma. Typically, thesurface tension of the covering layer thus treated, which has not yetbeen metallized, is in a range from 35 to 45 mN/m.

If necessary, the base layer may also be covered by multiple layers,i.e., in addition to the first covering layer described above, at leastone, possibly multiple intermediate layers made of polyolefins areattached below. This first intermediate layer generally contains atleast 80 weight-percent, preferably 95 to 100 weight-percent,particularly 98 to <100 weight-percent propylene homopolymer. Inaddition to this main component, the first intermediate layer maycontain typical additives such as stabilizers and/or neutralizationagents, as well as possibly pigments, such as TiO₂, in the particulareffective quantities. The thickness of the first intermediate layer isin a range from 4 to 10 μm, preferably 5 to 8 μm according to thepresent invention.

In principle, all materials described above for the first covering layermay be used as the polyolefins for the intermediate layer(s). Theselection of the polymers for the intermediate layer is, however,comparatively non-critical and, in addition to these describedhomopolymers or copolymers and terpolymers having a low comonomercomponent, other raw materials, particularly other mixed polymers areusable, which are typically employed in biaxially oriented films. Mixedpolymers of this type are described in detail in the following inconnection with the second intermediate layer and the second coveringlayer. However, isotactic propylene homopolymers having a melting pointof 155 to 165° C., preferably 160-162° C., and generally has a melt flowindex of 1 to 10 g/10 minutes, preferably 2 to 8 g/10 minutes, at 230°C. and a force of 21.6 N (DIN 53735) are preferred, these possiblepolymers.

Embodiments having a white first intermediate layer generally contain2-15 weight-percent, preferably 3-10 weight-percent TiO₂. Suitable TiO₂is described in detail in the following connection with the base layer.Pigmented intermediate layers of this type advantageously act as“visual” barriers and prevent the metal coating from showing through onthe diametrically opposite opaque side of the film and provide the filmon this opaque side with an advantageous white appearance.

The film according to the present invention is also distinguished byvacuoles in the base layer, which provide the film with an opaqueappearance. “Opaque film” as defined in the present invention means anopaque film, whose light transmission (ASTM-D 1003-77) is at most 70%,preferably at most 50%.

The base layer of the multilayer film contains polyolefin, preferably apropylene polymer, and vacuole-initiating fillers, as well as furthertypical additives as necessary in the particular effective quantities.In general, the base layer contains at least 70 weight-percent,preferably 75 to 98 weight-percent, particularly 85 to 95 weight-percentof the polyolefin, in relation to the weight of the layer in each case.In a further embodiment, the base layer may additionally containpigments, particularly TiO₂.

Propylene polymers are preferred as the polyolefins of the base layer.These propylene polymers contain 90 to 100 weight-percent, preferably 95to 100 weight-percent, particularly 98 to 100 weight-percent propyleneunits and have a melting point of 120° C. or higher, preferably 150 to170° C., and generally have a melt flow index of 1 to 10 g/10 minutes,preferably 2 to 8 g/10 minutes, at 230° C. and a force of 21.6 N (DIN53735). Isotactic propylene homopolymers having an atactic proportion of15 weight-percent or less, copolymers of ethylene and propylene havingan ethylene content of 5 weight-percent or less, copolymers ofpropylenes with C₄-C₈ olefins having an olefin content of 5weight-percent or less, terpolymers of propylene, ethylene, and butylenehaving an ethylene content of 10 weight-percent or less and having abutylene content of 15 weight-percent or less are preferred propylenepolymers for the base layer, isotactic propylene homopolymer beingespecially preferred. The weight-percents specified relate to theparticular polymer.

Furthermore, a mixture of the cited propylene homopolymers and/orcopolymers and/or terpolymers and other polyolefins, particularly madeof monomers having 2 to 6 C atoms, is suitable, the mixture containingat least 50 weight-percent, particularly at least 75 weight-percentpropylene polymer. Suitable other polyolefins in the polymer mixture arepolyethylenes, particularly HDPE, MDPE, LDPE, VLDPE, and LLDPE, theproportion of these polyolefins not exceeding 15 weight-percent each, inrelation to the polymer mixture.

The opaque base layer of the film generally contains vacuole-initiatingfillers in a quantity of at most 30 weight-percent, preferably 2 to 25weight-percent, particularly 2 to 15 weight-percent, in relation to theweight of the opaque base layer.

As defined in the present invention, vacuole-initiating fillers aresolid particles which are incompatible with the polymer matrix andresult in the formation of vacuole-like cavities when the film isstretched, the size, type, and number of the vacuoles being a functionof the quantity and size of the solid particles and the stretchingconditions such as the stretching ratio and stretching temperature. Thevacuoles reduce the density and provide the films with a characteristicnacreous, opaque appearance, which arises due to light scattering at theboundaries “vacuole/polymer matrix”. The light scattering at the solidparticles themselves generally contributes comparatively little to theopacity of the film. Typically, the vacuole-initiating fillers have aminimum size of 1 μm, in order to result in an effective, i.e.,opaque-making quantity of vacuoles. In general, the average particlediameter of the particles is 1 to 6 μm, preferably 1 to 4 μm. Thechemical character of the particles plays a subordinate role.

Typical vacuole-initiating fillers are inorganic and/or organicmaterials which are incompatible with polypropylene, such as aluminumoxide, aluminum sulfate, barium sulfate, calcium carbonate, magnesiumcarbonate, silicates such as aluminum silicate (kaolin clay) andmagnesium silicate (talcum) and silicon dioxide, of which calciumcarbonate and silicon dioxide are preferably used. The typically usedpolymers which are incompatible with the polymers of the base layer comeinto consideration as organic fillers, particularly copolymers of cyclicolefins (COC) as described in EP-A-O 623 463, polyesters, polystyrenes,polyamides, and halogenated organic polymers, with polyesters such aspolybutylene terephthalate and cycloolefinic copolymers being preferred.Incompatible materials and/or incompatible polymers means, as defined inthe present invention, that the material and/or the polymer exists inthe film as separate particles and/or as a separate phase.

In a further embodiment, the base layer may additionally containpigments, for example, in a quantity of 0.5 to 10 weight-percent,preferably 1 to 8 weight-percent, particularly 1 to 5 weight-percent.The specifications relate to the weight of the base layer.

As defined in the present invention, pigments are incompatible particleswhich essentially do not result in vacuole formation upon stretching ofthe film. The coloring effect of the pigments is caused by the particlesthemselves. The term “pigments” is generally connected to an averageparticle diameter in the range from 0.01 to at most 1 μm and includesboth “white pigments”, which color the film white, and also “colorpigments”, which provide the film with a colored or black color. Ingeneral, the average particle diameter of the pigments is in the rangefrom 0.01 to 1 μm, preferably 0.01 to 0.7 μm, particularly 0.01 to 0.4μm.

Typical pigments are materials such as aluminum oxide, aluminum sulfate,barium sulfate, calcium carbonate, magnesium carbonate, silicates suchas aluminum silicate (kaolin clay) and magnesium silicate (talcum),silicon dioxide, and titanium dioxide, of which white pigments such ascalcium carbonate, silicon dioxide, titanium dioxide, and barium sulfateare preferably used. Titanium dioxide is especially preferred. Variousmodifications and coatings of TiO₂ are known per se in the related art.

The density of the film is essentially determined by the density of thebase layer. The density of the vacuole-containing base layer isgenerally reduced by the vacuoles, if larger quantities of TiO₂ do notcompensate for the density-reducing effect of the vacuoles. In general,the density of the opaque base layer is in a range from 0.45-0.85 g/cm³.The density of the film may vary in a wide range for the white-opaqueembodiments described and is generally in a range from 0.5 to 0.95g/cm³, preferably 0.6 to 0.9 g/cm³. The density is elevated in principleby adding TiO₂, but simultaneously reduced by the vacuole-initiatingfillers in the base layer. For a base layer which does not contain anydensity-elevating TiO₂, the density of the opaque base layer ispreferably in a range from 0.45 to 0.75 g/cm³, while in contrast therange from 0.6 to 0.9 g/cm³ is preferred for the white-opaque baselayer.

The total thickness of the film is generally in a range from 20 to 100μm, preferably 25 to 60 μm, particularly 30 to 50 μm. The thickness ofthe base layer is correspondingly 10 to 50 ∞m, preferably 10 to 40 μm.

In a further preferred embodiment, the film includes even furtherlayers, which are applied to the diametrically opposite side of the baselayer. Through a second covering layer, four-layer films result.Embodiments which additionally have a second intermediate layer and asecond covering layer applied thereto result in five-layer films. Inthese embodiments, the thickness of the second covering layer isgenerally 0.5-3 μm, intermediate layers are in the range from 1 to 8 μm.Combinations made of intermediate layer and covering layeradvantageously have a total thickness of 2 to 8 μm. Sealable layers arepreferred as further layers, both layers which may be hot sealed andthose which may be cold sealed being understood here. Cold seal coatingsmay also be applied directly to the surface of the base layer. Ingeneral, however, it is preferable to first cover the base layer withthe polymer covering layer and apply the cold seal coating to thispolymer covering layer.

The additional covering layer and intermediate layer generally containat least 80 weight-percent, preferably 90 to <100 weight-percentolefinic polymers or mixtures thereof. Suitable polyolefins are, forexample, polyethylenes, propylene copolymers, and/or propyleneterpolymers, as well as the propylene homopolymers already described inconnection with the base layer.

Suitable propylene copolymers or terpolymers are generally synthesizedfrom at least 50 weight-percent propylene and ethylene and/or butyleneunits as the comonomers. Preferred mixed polymers are randomethylene-propylene copolymers having an ethylene content of 2 to 10weight-percent, preferably 5 to 8 weight-percent, or randompropylene-butylene-1 copolymers, having a butylene content of 4 to 25weight-percent, preferably 10 to 20 weight-percent, each in relation tothe total weight of the copolymers, or randomethylene-propylene-butylene-1 terpolymers, having an ethylene content of1 to 10 weight-percent, preferably 2 to 6 weight-percent, and abutylene-1 content of 3 to 20 weight-percent, preferably 8 to 10weight-percent, each in relation to the total weight of the terpolymers.These copolymers and terpolymers generally have a melt flow index of 3to 15 g/10 minutes, preferably 3 to 9 g/10 minutes (230° C., 21.6 N DIN53735) and a melting point of 70 to 145° C., preferably 90 to 140° C.(DSC).

Suitable polyethylenes are, for example, HDPE, MDPE, LDPE, VLDPE, andLLDPE, of which HDPE and MDPE types are especially preferred. The HDPEgenerally has an MFI (50 N/190° C.) of >0.1 to 50 g/10 minutes,preferably 0.6 to 20 g/10 minutes, measured according to DIN 53 735, anda coefficient of viscosity, measured according to DIN 53728, part 4, orISO 1191, in the range from 100 to 450 cm³/g, preferably 120 to 280cm³/g. The crystallinity is 35 to 80%, preferably 50 to 80%. Thedensity, measured at 23° C. according to DIN 53 479, method A, or ISO1183, is in the range from >0.94 to 0.96 g/cm³. The melting point,measured using DSC (maximum of the melting curve, heating speed 20°C./minute), is between 120 and 140° C. Suitable MDPE generally has anMFI (50 N/190° C.) of >0.1 to 50 g/10 minutes, preferably 0.6 to 20 g/10minutes, measured according to DIN 53 735. The density, measured at 23°C. according to DIN 53 479, method A, or ISO 1183, is in the rangefrom >0.925 to 0.94 g/cm³. The melting point, measured using DSC(maximum of the melting curve, heating speed 20° C./minute), is between115 and 130° C.

In regard to the appearance of this film side, embodiments having apropylene homopolymer intermediate layer and a sealable covering layerare preferred. In this case, the intermediate layer is synthesized fromat least 80 weight-percent, preferably 85 to 98 weight-percent propylenehomopolymer and has a thickness of at least 2 μm, preferably 2.5 to 6μm. To improve the appearance, particularly the degree of whiteness, thepigments described above for the base layer are added to thisintermediate layer, particularly TiO₂ in a quantity of 2 to 12weight-percent, preferably 3 to 8 weight-percent, in relation to theweight of the intermediate layer.

In general, sealing layers are applied to intermediate layers coloredwhite in this way in a thickness of 0.3 to 4 μm. Typical sealing layersmade of propylene copolymers or propylene terpolymers come intoconsideration for this purpose. Suitable propylene copolymers orterpolymers are generally synthesized from at least 50 weight-percentpropylene and ethylene and/or butylene units as the comonomers. Randomethylene-propylene copolymers having an ethylene content of 2 to 10weight-percent, preferably 5 to 8 weight-percent, or randompropylene-butylene-1 copolymers, having a butylene content of 4 to 25weight-percent, preferably 10 to 20 weight-percent, each in relation tothe total weight of the copolymers, or randomethylene-propylene-butylene-1 terpolymers, having an ethylene content of1 to 10 weight-percent, preferably 2 to 6 weight-percent, and abutylene-1 content of 3 to 20 weight-percent, preferably 8 to 10weight-percent, each in relation to the total weight of the terpolymers,are preferred. These copolymers and terpolymers generally have a meltflow index of 3 to 15 g/10 minutes, preferably 3 to 9 g/10 minutes (230°C., 21.6 N DIN 53735) and a melting point of 70 to 145° C., preferably90 to 140° C. (DSC).

These embodiments are distinguished by an especially advantageousappearance on the side diametrically opposite the metal coating. Theaddition of titanium dioxide effectively prevents the metal coating fromshowing through, due to which this “opaque” side of the film appearsgrayish and impairs the white appearance.

If the film is used as a package for chocolate products, either themetallized side (after application of an adhesion promoter) or thesurface of the “opaque side” is provided with a cold seal adhesive. Inaddition, the film may be used as a normal sealable film in which themanufacture of the package is performed via hot sealing.

If necessary, the film may also be used as a pouch package for powderedbulk products. For applications of this type, a mixture made of thedescribed propylene copolymers and/or terpolymers and the citedpolyethylenes is especially used for the second intermediate layer and,if necessary, for the second covering layer. These mixtures areespecially advantageous in regard to the sealing properties of the filmif the pouch is used for packaging powdered bulk products. Using thecurrent methods for packaging powders, contamination of the seal regionsmay not be effectively prevented. These contaminations frequently resultin problems during sealing. The seal seams have reduced or even nostrength in the contaminated regions, and the tightness of the seal seamis also impaired. Surprisingly, the contaminations interfere onlyslightly or not at all during sealing if the seal layers are synthesizedfrom a mixture of propylene polymers and polyethylenes. Covering layermixtures which contain HDPE and/or MDPE, having an HDPE or MDPEproportion of 10 to 50 weight-percent, particularly 15 to 40weight-percent, are especially advantageous for this purpose.

In a further application, the film according to the present inventionmay be processed into a laminate. For this purpose, the metallized sideis preferably laminated against an opaque or transparent polypropyleneor polyethylene film. This composite is preferably used for packagingfatty foods, e.g., dry powders or snacks.

As already noted, all layers of the film preferably containneutralization agents and stabilizers in the particular effectivequantities.

The typical stabilizing compounds for ethylene, propylene, and otherolefin polymers may be used as stabilizers. The quantity added isbetween 0.05 and 2 weight-percent. Phenolic stabilizers,alkaline/alkaline earth stearates, and alkaline/alkaline earthcarbonates are especially suitable. Phenolic stabilizers are preferredin a quantity of 0.1 to 0.6 weight-percent, particularly 0.15 to 0.3weight-percent, and having a molar mass of more than 500 g/mol.Pentaerythrityl-tetrakis-3-(3,5-di-tertiarybutyl-4-hydroxyphenyl)-propionate or1,3,5-trimethyl-2,4,6-tris(3,5-di-tertiary butyl-4-hydroxybenzyl)benzeneare especially advantageous.

Neutralization agents are preferably calcium stearate, and/or calciumcarbonate and/or synthetic dihydrotalcite (SHYT) of an average particlesize of at most 0.7 μm, an absolute particle size of less than 10 μm,and a specific surface area of at least 40 m²/g. In general,neutralization agents are used in a quantity of 50 to 1000 ppm, inrelation to the layer.

In a preferred embodiment, antiblocking agents are added to both thecovering layer to be metallized and also the diametrically oppositecovering layer.

Suitable antiblocking agents are inorganic additives such as silicondioxide, calcium carbonate, magnesium silicate, aluminum silicate,calcium phosphate, and the like, and/or incompatible polymers such aspolymethyl methacrylate (PMMA) polyamides, polyesters, polycarbonates,with polymethyl methacrylate (PMMA), silicon dioxide, and carbon dioxidebeing preferred. The effective quantity of antiblocking agent is in therange from 0.1 to 2 weight-percent, preferably 0.1 to 0.5weight-percent, in relation to the particular covering layer. Theaverage particle size is between 1 and 6 μm, particularly 2 and 5 μm,particles having a spherical shape, as described in EP-A-0 236 945 andDE-A-38 01 535, being especially suitable.

Furthermore, the present invention relates to methods for manufacturingthe multilayer film according to the present invention according tocoextrusion methods known per se, the tentering method beingparticularly preferred.

In the course of this method, the melts corresponding to the individuallayers of the film are coextruded through a sheet die, the film thusobtained is drawn off to solidify on one or more roll(s), the film issubsequently stretched (oriented), and the stretched film is thermallyfixed and possibly plasma, corona, or flame treated on the surface layerprovided for treatment.

Specifically, for this purpose, as is typical in the extrusion methods,the polymers and/or the polymer mixture of the individual layers iscompressed in an extruder and liquefied, the vacuole-initiating fillersand other possibly added additives already being able to be contained inthe polymer and/or in the polymer mixture. Alternatively, theseadditives may also be incorporated via a masterbatch.

The melts are then pressed jointly and simultaneously through a sheetdie, and the multilayered film extruded is drawn off on one or moredraw-off rolls at a temperature of 5 to 100° C., preferably 10 to 50°C., so that it cools and solidifies.

The film thus obtained is then stretched longitudinally and transverselyto the extrusion direction, which results in orientation of themolecular chains. The longitudinal stretching is preferably performed ata temperature of 80 to 150° C., expediently with the aid of two rollsrunning at different speeds in accordance with the stretching ratiodesired, and the transverse stretching is preferably performed at atemperature of 120 to 170° C. with the aid of a corresponding tenterframe. The longitudinal stretching ratios are in the range from 4 to 8,preferably 4.5 to 6. The transverse stretching ratios are the range from5 to 10, preferably 7 to 9.

The stretching of the film is followed by its thermal fixing (heattreatment), the film being held approximately 0.1 to 10 seconds long ata temperature of 100 to 160° C. Subsequently, the film is wound up in atypical way using a winding device.

Preferably, after the biaxial stretching, one or both surfaces of thefilm is/are plasma, corona, or flame treated according to one of theknown methods. The treatment intensity is generally in the range from 35to 50 mN/m, preferably 37 to 45 mN/m, particularly 39 to 40 mN/m.

For the corona treatment, the film is guided between two conductorelements used as electrodes, such a high voltage being applied betweenthe electrodes, usually alternating voltage (approximately 10,000 V and10,000 Hz), that spray or corona discharges may occur. Through the sprayor corona discharge, the air above the film surface is ionized andreacts with the molecules of the film surface, so that polarintercalations arise in the essentially nonpolar polymer matrix. Thetreatment intensities are within the typical scope, 37 to 45 mN/m beingpreferred.

The coextruded multilayered film is provided on the outer surface of thefirst covering layer with a metal coating, preferably made of aluminum,according to methods known per se. This metallization is performed in avacuum chamber in which aluminum is vaporized and deposited on the filmsurface. In a preferred embodiment, the surface to be metallized issubjected to plasma treatment directly before the metallization. Thethickness of the metal coating generally correlates with the opticaldensity of the metallized film, i.e., the thicker the metal coating is,the higher the optical density of the metallized film. In general, theoptical density of the metallized film according to the presentinvention is to be at least 2, particularly 2.5 to 4.

The following measurement methods were used to characterize the rawmaterials and the films:

Melt-Flow Index

The melt-flow index was measured according to DIN 53735 at 21.6 N loadand 230° C.

Optical Density

The optical density is the measurement of the transmission of a definedlight beam. The measurement was performed using a densitometer of thetype TCX from Tobias Associates Inc. The optical density is a relativevalue which is specified without a dimension.

Water Vapor and Oxygen Permeability

The water vapor permeability was determined in accordance with DIN 53122part 2. The oxygen barrier effect was determined in accordance with thedraft of DIN 53380 part 3 at an ambient humidity of approximately 50%.

Determination of the Ethylene Content

The ethylene content of the copolymer was determined using ¹³C NMRspectroscopy. The measurements were performed using an atomic resonancespectrometer from Bruker Avance 360. The copolymer to be characterizedwas dissolved in tetrachloroethane, so that a 10% mixture resulted.Octamethyl tetrasiloxane (OTMS) was added as a reference standard. Theatomic resonance spectrum was measured at 120° C. The spectra wereanalyzed as described in J. C. Randall Polymer Sequence Distribution(Academic Press, New York, 1977).

Melting Point and Melting Enthalpy

The melting point and the melting enthalpy were determined using DSC(differential scanning calorimetry) measurement (DIN 51 007 and DIN 53765). Several milligrams (3 to 5 mg) of the raw material to becharacterized were heated in a differential calorimeter at a heatingspeed of 20° C. per minute. The thermal flux was plotted against thetemperature and the melting point was determined as the maximum of themelting curve and the melting enthalpy was determined as the area of theparticular melting peak.

Density

The density was determined according to DIN 53 479, method A.

Surface Tension

The surface tension was determined via the ink method according to DIN53364.

The present invention will now be explained through the followingexamples.

EXAMPLE 1

A five-layer precursor film was extruded according to the coextrusionmethod from a sheet die at 240 to 270° C. This precursor film was firstdrawn off on a cooling roll and cooled. Subsequently, the precursor filmwas oriented in the longitudinal and transverse directions and finallyfixed. The surface of the first covering layer was pretreated usingcorona to elevate the surface tension. The five-layer film had a layerstructure of first covering layer/first intermediate layer/baselayer/second intermediate layer/second covering layer. The individuallayers of the film had the following composition:

First covering layer (0.5 μm):

˜100 weight-percent ethylene-propylene copolymer having an ethyleneproportion of 1.7 weight-percent (in relation to the copolymer) and amelting point of 155° C.; and a melt flow index of 8.5 g/10 minutes at230° C. and 2.16 kg load (DIN 53 735) and a melting enthalpy of 96.9 J/g

First intermediate layer (6.5 μm)

˜100 weight-percent propylene homopolymer (PP) having ann-heptane-soluble proportion of approximately 4 weight-percent (inrelation to 100% PP) and a melting point of 163° C.; and a melt flowindex of 3.3 g/10 minutes at 230° C. and 2.16 kg load (DIN 53 735)

Base layer:

91.6 weight-percent propylene homopolymer (PP) having ann-heptane-soluble proportion of approximately 4 weight-percent (inrelation to 100% PP) and a melting point of 163° C.; and a melt flowindex of 3.3 g/10 minutes at 230° C. and 2.16 kg load (DIN 53 735) and6.0 weight-percent calcium carbonate, average particle diameterapproximately 2.7 μm 2.4 weight-percent titanium dioxide, averageparticle diameter of 0.1 to 0.3 μm

Second intermediate layer (3 μm)

96.4 weight-percent propylene homopolymer (PP) having ann-heptane-soluble proportion of approximately 4 weight-percent (inrelation to 100% PP) and a melting point of 163° C.; and a melt flowindex of 3.3 g/10 minutes at 230° C. and 2.16 kg load (DIN 53 735) and3.6 weight-percent titanium dioxide, average particle diameter of 0.1 to0.3 μm

Second covering layer (0.7 μm):

99.7 weight-percent ethylene-propylene copolymer having an ethyleneproportion of 4 weight-percent (in relation to the copolymer) and amelting point of 136° C.; and a melt flow index of 7.3 g/10 minutes at230° C. and 2.16 kg load (DIN 53 735) and a melting enthalpy of 64.7 J/g

0.1 weight-percent antiblocking agent having an average particlediameter of approximately 4 μm (Sylobloc 45)

All layers of the film additionally contained stabilizers andneutralization agents in typical quantities.

Specifically, the following conditions and temperatures were selectedwhen manufacturing the film: extrusion: extrusion temperature approx.250-270° C. cooling roll: temperature 30° C. longitudinal stretching: T= 120° C. longitudinal stretching by a factor of 5 T = 160° C.transverse stretching: transverse stretching by a factor of 9 T = 100°C. fixing:

The film was surface treated on the surface of the first covering layerusing corona and has a surface tension of 38 mN/m. The film has athickness of 35 μm and an opaque appearance.

EXAMPLE 2

A film was manufactured according to example 1. In contrast to example1, the second intermediate layer contained no TiO₂. The compositions ofthe remaining layers and the manufacturing conditions were not changed.

COMPARATIVE EXAMPLE 1

An opaque film was manufactured according to example 1. In contrast toexample 1, the first intermediate layer was left out, i.e., the firstcovering layer was applied directly to the surface of the base layer.

COMPARATIVE EXAMPLE 2

An opaque film was manufactured according to example 1. In contrast toexample 1, a typical propylene copolymer was used in the first coveringlayer:

First covering layer (0.5 μm):

˜100 weight-percent ethylene-propylene copolymer having an ethyleneproportion of 4 weight-percent (in relation to the copolymer) and amelting point of 136° C.; and a melt flow index of 7.3 g/10 minutes at230° C. and 2.16 kg load (DIN 53 735) and a melting enthalpy of 64.7 J/g

COMPARATIVE EXAMPLE 3

A film was manufactured as an example 2. In contrast to example 2, thebase layer contained no vacuole-initiating fillers and no TiO₂ and thesecond intermediate layer also contained no TiO₂. A three-layer filmresulted, since the intermediate layers and the base layer were onlymade of propylene homopolymer.

All films according to the examples and the comparative examples werecoated with an aluminum coating in a vacuum metallizing facility. Toimprove the metal adhesion, the surface was subjected to a plasmatreatment directly before the coating. The properties of the metallizedfilms according to the examples in the comparative examples aresummarized in Table 1. It has been shown that the films according to thepresent invention according to examples 1 and 2 have outstanding barriervalues against water vapor and oxygen and, simultaneously, a good opaqueand/or white appearance on the diametrically opposite side. Density WDD38° C. OTR 23° C., Thick- of the 90% 50% ness film relative relativeExample μm g/cm³ Appearance humidity*** humidity*** Example 1 35 0.71 ++0.15 9 Example 2 35 0.71 + 0.14 10 CE 1 35 0.71 ++ 0.5 >100 CE 2 35 0.71++ 0.5 135 CE 3 35 0.91 transparent 0.3 30* qualitative judgment of showing through of metal coating on thediametrically opposite side***after metallization

1. A metallized, coextruded multilayered biaxially orientedpolypropylene multilayer film, which as a vacuole-containing base layer,this vacuole-containing base layer being covered by one or more layersand a thickness of this cover layer or layers being a total of at least3 μm and the film being metallized on the outer surface of this coverlayer or layers and the metallized film having a water vaporpermeability ≦0.5 g/m²*day at 38° C. and 90% relative ambient humidityand an oxygen permeability of ≦50 cm³/m²*day*bar.
 2. The film accordingto claim 1, characterized in that a cover layer is applied as a coveringlayer to the base layer.
 3. The film according to claim 1, characterizedin that an intermediate layer and a first covering layer are applied ascover layers to the base layer.
 4. The film according to claim 1,characterized in that the covering layer and/or the intermediate layercontain at least 80 weight-percent propylene homopolymers, ethylenehomopolymers, propylene copolymers having less than 3 weight-percentcomonomers or ethylene copolymers having less than 3 weight-percentcomonomers or a mixture of these polymers.
 5. The film according toclaim 4, characterized in that the comonomer of the propylene copolymeris ethylene or butylene.
 6. The film according to claim 5, characterizedin that the propylene copolymer is a propylene-ethylene copolymer havingan ethylene content of <2.5 weight-percent and a melting point of 150 to160° C.
 7. The film according to claim 4, characterized in that thepropylene homopolymer is an isotactic propylene homopolymer having amelting point of 159 to 162° C. or a highly isotactic propylenehomopolymer having an isotacticity of more than 97% or a propylenehomopolymer manufactured using metallocene catalysts.
 8. The filmaccording to claim 1, characterized in that the base layer issynthesized from propylene homopolymer and contains 2 to 15weight-percent vacuole-initiating fillers and has a density of 0.45-0.85cm³/g.
 9. The film according to claim 1, characterized in that theoptical density of the metallized film is at least 2.0.
 10. The filmaccording to claims 9, characterized in that the optical density is 2.5to
 5. 11. The film according to claim 10, characterized in that the filmcontains a second covering layer made of propylene homopolymer,polyethylene homopolymer, or propylene copolymer and/or propyleneterpolymer.
 12. The film according to claim 11, characterized in thatthe second covering layer is sealable and has a thickness of 0.3 to 4μm.
 13. The film according to claim 12, characterized in that a secondintermediate layer is attached between the base layer and the secondcovering layer.
 14. The film according to claim 2, characterized in thatthe first covering layer has a thickness of 4 to 8 μm.
 15. The filmaccording to claim 3, characterized in that the intermediate layer has athickness of at least 3.5 μm and the first covering layer has athickness of at least 0.5 μm.
 16. The film according to claim 15,characterized in that the intermediate layer has a thickness of at least4 to 10 μm and the covering layer has a thickness of at least 0.8 to 3μm.
 17. A process for manufacturing a package which comprises using thefilm according to claim 1 and wherein the package has a barrier inrelation to water vapor and oxygen.
 18. (cancelled)
 19. A method formanufacturing a film according to claim 1, treating the surface of theopaque film to be metallized using plasma directly before themetallization.
 20. A process for manufacturing a laminate whichcomprises laminating the metallized side of the film according to claim1 against a further polypropylene film or against a polyethylene film.21. A pouch which comprises the film as claimed in claim 1 and apowdered bulk product.